Ion implanters are used to implant or "dope" silicon wafers with impurities to produce n or p type extrinsic materials. The n and p type extrinsic materials are utilized in the production of semiconductor integrated circuits. As its name implies, the ion implanter dopes the silicon wafers with a selected ion species to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in n type extrinsic material wafers. If p type extrinsic material wafers are desired, ions generated with source materials such as boron, gallium or indium will be implanted.
The ion implanter includes an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and accelerated along a predetermined beam path to an implantation station. The ion implanter includes beam forming and shaping structure extending between the ion source and the implantation station. The beam forming and shaping structure maintains the ion beam and bounds an elongated interior cavity or region through which the beam passes en route to the implantation station. When operating the implanter, the interior region must be evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with air molecules.
For high current ion implanters, the wafers at the implantation station are mounted on a surface of a rotating support. As the support rotates, the wafers pass through the ion beam. Ions traveling along the beam path collide with and are implanted in the rotating wafers. A robotic arm withdraws wafers to be treated from a wafer cassette and positions the wafers on the wafer support surface. After treatment, the robotic arm removes the wafers from the wafer support surface and redeposits the treated wafers in the wafer cassette.
Operation of an ion implanter results in the production of certain contaminant particles. One source of contaminant particles is undesirable species of ions generated in the ion source. Contaminant particles with respect to a given implant result from the presence of residual ions from a previous implant in which different ions were implanted. For example, after implanting boron ions in a given number of wafers, it may be desired to change over the implanter to implant arsenic ions. It is likely that some residual boron atoms remain in the interior region of the implanter.
Yet another source of contaminant particles is photoresist material. Photoresist material is coated on wafer surfaces prior to implantation and is required to define circuitry on the completed integrated circuit. As ions strike the wafer surface, particles of photoresist coating are dislodged from the wafer.
Contaminant particles which collide with and adhere to wafers during ion treatment are a major source of yield loss in the fabrication of semiconductor and other devices which require submicroscopic pattern definition on the treated wafers.
In addition to rendering the implanted or treated wafers unsuitable for their intended purpose in the manufacture of integrated circuits, contaminant particles adhering to interior surfaces of the ion implanter reduce the efficiency of ion implanter components, for example, the performance of an ion beam neutralization apparatus will be detrimentally effected by a build-up of photoresist particles on the apparatus' aluminum extension tube.
The vacuum environment of the implanter interior makes capture and removal of contaminant particles problematical. In a vacuum, the motion of submicroscopic particles is extremely difficult to control, particle movement is controlled to a great extent by electrostatic forces. Gravitational forces become insignificant with decreasing particle size.
It has been found that particles moving within the evacuated interior of the implanter bounce or rebound numerous times before settling on and adhering to the workpiece surface or to an interior surface of the implanter. Experience indicates that such a moving particle may bounce 10 to 25 times before settling.
In essence, a particle collector includes a particle adhering surface. Particles colliding with the surface become attached thereto and are removed when the collector is removed. However, for a particle collector to be used in conjunction with an ion implanter, the particle collector would have to be compatible with the vacuum environment. Conventional particle collector surfaces, e.g., adhesives, porous materials, oily materials, etc. tend to outgas in a vacuum environment which makes them inappropriate for use in the implanter. What is needed is a particle collector for contaminant particles which is suitable for use in a vacuum environment and which exhibits high particle adhering qualities.